Dietary linoleic acid lowering alone does not lower arachidonic acid or endocannabinoids among women with overweight and obesity: A randomized, controlled trial

Abstract

The linoleic acid (LA)-arachidonic acid (ARA)-inflammatory axis suggests dietary LA lowering benefits health because it lowers ARA and ARA-derived endocannabinoids (ECB). Dietary LA reduction increases concentrations of omega-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and DHA derived ECB. The aim of this study was to examine targeted reduction of dietary LA, with and without EPA and DHA, on plasma EPA and DHA and ECB (2-arachidonoyl glycerol [2-AG], anandamide [AEA], and docosahexaenoyl ethanolamide [DHA-EA]). Healthy, pre-menopausal women (n = 62, BMI 30 ± 3 kg/m² , age 35 ± 7 years; mean ± SD) were randomized to three 12-week controlled diets: (1) high LA, low omega-3 EPA and DHA (H6L3); (2) low LA, low omega-3 EPA and DHA (L6L3); or (3) low LA, high omega-3 EPA and DHA (L6H3). Baseline plasma fatty acids and ECB were similar between diets. Starting at 4 weeks, L6L3 and L6H3 lowered plasma LA compared to H6L3 (p < 0.001). While plasma ARA changed from baseline by 8% in L6L3 and -8% in L6H3, there were no group differences. After 4 weeks, plasma EPA and DHA increased from baseline in women on the L6H3 diet (ps < 0.001) and were different than the H6L3 and L6L3 diets. No differences were found between diets for AEA or 2-AG, however, in L6L3 and L6H3, AEA increased by 14% (ps < 0.02). L6H3 resulted in 35% higher DHA-EA (p = 0.013) whereas no changes were seen with the other diets. Lowering dietary LA did not result in the expected changes in fatty acids associated with the LA-ARA inflammatory axis in women with overweight and obesity.

Keywords: arachidonic acid; diet; docosahexaenoic acid; eicosapentaeonic acid; endocannabinoid; linoleic acid.

Figure 1.

PUFA Biosynthetic Pathway. LA (18:2n-6) and aLNA (18:3n-3) are the most common PUFA in diet and can be converted to longer chain PUFA such as ARA (20:4n-6), EPA (20:5n-3) and DHA (22:6n-3). The most commonly reported pathway for this conversion has LA and aLNA being desaturated by Delta-6 desaturase. After the initial delta 6 desaturation, PUFA are elongated, desaturated (delta-5 desaturase) and then elongated 2 more times resulting in 24 carbon n-3 and n-6 products. These 24 carbon PUFA then are acted on by the delta-6 desaturase and then oxidized to 22 carbon products, particularly DHA. For each step in this pathway, there is competition between n-3 and n-6 PUFA and there is additional competition for the delta-6 desaturase as it catalyzes two reactions in the pathway.

Figure 2.

Consort flow diagram. Abbreviations: high LA, low n-3 (H6L3); low LA, low n-3 (L6L3); and low LA, high n-3 (L6H3).

Figure 3.

Plasma fatty acids at baseline, 4, 8 and 12 weeks in women consuming three dietary interventions varying in linoleic acid (LA) and omega-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Linear mixed modeling (SAS PROC MIXED) was used to analyze outcome data, with diet group (high LA, low n-3 EPA and DHA (H6L3); low LA, low n-3 EPA and DHA (L6L3); and low LA, high n-3 EPA and DHA (L6H3)) as the between-subjects factor and study timepoint (baseline, week 4, week 8 and week 12) as the within-subject factor. Fatty acids were measured as percent of total fatty acids (wt%). Significance was set at p < 0.05 for all tests, and all post hoc comparisons of least square means were conducted using Sidak’s correction. Abbreviations: n-6, omega-6; highly unsaturated fatty acids, HUFA; fatty acids, FA; LA, linoleic acid. *p<0.05.

Figure 4.

Percent change in fatty acids from baseline to 12 weeks in women consuming three dietary interventions varying in linoleic acid (LA) and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Wilcoxon signed rank test was used to analyze percent change from baseline within-subjects in the three diet groups (high LA, low n-3 EPA and DHA (H6L3); low LA, low n-3 EPA and DHA (L6L3); and low LA, high n-3 EPA and DHA (L6H3)). Fatty acids were measured as percent of total fatty acids (wt%). Significance was set at p<0.05 for all tests*p<0.05, **p<0.01, ***p<0.001, ****p<0.0001. Abbreviations: HUFA, highly unsaturated fatty acids; n-6, omega-6.

Figure 5.

Plasma endocannabinoids at baseline and 12 weeks in women consuming three dietary interventions varying in omega-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and linoleic acid (LA). with diet group (high LA, low n-3 EPA and DHA (H6L3); low LA, low n-3 EPA and DHA (L6L3); and low LA, high n-3 EPA and DHA (L6H3)) as the between-subjects factor and study timepoint (baseline and week 12) as the within-subject factor. Endocannabinoids were measured in ng/mL. Significance was set at p<0.05 for all tests, and all post hoc comparisons of least square means were conducted using Sidak’s correction. Abbreviations: AEA, N-arachidonoylethanolamine; 2AG, 2-arachadonoylglycerol; DHA-EA docosahexanoylethanolamine;.

Figure 6.

Percent change in endocannabinoids from baseline to 12 weeks in women consuming three dietary interventions varying in linoleic acid and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Wilcoxon signed rank test was used to analyze percent change from baseline within-subjects in the three diet groups (high LA, low n-3 EPA and DHA (H6L3); low LA, low n-3 EPA and DHA (L6L3); and low LA, high n-3 EPA and DHA (L6H3)). Endocannabinoids were measured in ng/mL. Significance was set at p<0.05 for all tests. *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001